How to bake "full render" in blender
Select the high poly then the low poly mesh. So the active will be the low poly. Check the option Selected to Active in the Bake panel. Here is an example:  



To be able to do this you have to UV unwrap the plane or cuboid and assign
a texture for the UV to bake on. But you have already done this. :) To use
a cubeoid, you simply have to adjust it so that it represents the models
geometry as close as possible.